The three-dimensional (3D) crystal structures of the GAF3 domain of
cyanobacteriochrome Slr1393 (Synechocystis PCC6803)
carrying a phycocyanobilin chromophore could be solved in both 15-Z
dark-adapted state, Pr, λmax = 649 nm, and 15-E
photoproduct, Pg, λmax = 536 nm (resolution, 1.6 and 1.86
Å, respectively). The structural data allowed identifying the large spectral shift
of the Pr-to-Pg conversion as resulting from an out-of-plane rotation of the chromophore’s
peripheral rings and an outward movement of a short helix formed from a formerly
unstructured loop. In addition, a third structure (2.1-Å resolution) starting
from the photoproduct crystals allowed identification of elements that regulate
the absorption maxima. In this peculiar form, generated during X-ray exposition,
protein and chromophore conformation still resemble the photoproduct state, except
for the D-ring already in 15-Z configuration and tilted
out of plane akin the dark state. Due to its formation from the photoproduct,
it might be considered an early conformational change initiating the parental
state-recovering photocycle. The high quality and the distinct features of the
three forms allowed for applying quantum-chemical calculations in the framework
of multiscale modeling to rationalize the absorption maxima changes. A systematic
analysis of the PCB chromophore in the presence and absence of the protein environment
showed that the direct electrostatic effect is negligible on the spectral tuning.
However, the protein forces the outer pyrrole rings of the chromophore to deviate
from coplanarity, which is identified as the dominating factor for the color regulation.

The nonlinear processes of frequency conversion such as second harmonic
generation (SHG) usually obey certain selection rules, resulting from the preservation
of different kinds of physical quantities, e.g. the angular momentum. For the
SHG created by a monolayer of transition-metal dichalcogenides (TMDCs) such as
WS2, the valley-exciton locked selection rule predicts an
SHG signal in the cross-polarization state. By combining plasmonic nanostructures
with a monolayer of TMDC, a hybrid metasurface is realized, which affects this
nonlinear process because of an additional polarization conversion process. Here,
we observe that the plasmonic metasurface modifies the light-matter interaction
with the TMDC, resulting in an SHG signal that is co-polarized with respect to
the incident field, which is usually forbidden for the monolayers of TMDC. We
fabricate such hybrid metasurfaces by placing plasmonic nanorods on top of a monolayer
WS2 and study the valley-exciton locked SHG emission from
such system for different parameters, such as wavelength and polarization. Furthermore,
we show the potential of the hybrid metasurface for tailoring nonlinear processes
by adding additional phase information to the SHG signal using the Pancharatnam-Berry
phase effect. This allows direct tailoring of the SHG emission to the far-field.